This invention relates to the field of laser-radar imaging and related sensor technology.
Conventional active laser-radar imaging systems provide an array of sensor elements which combine optically to capture an image of a body or target. The use of an active or pulsed light source such as a laser to illuminate the body or target generally provides an improved optical return, thereby allowing a three dimensional image of a scene to be captured. Such images comprise information relating to azimuth, elevation and range.
Where a laser is used as the active or pulsed light source to produce the required optical returns, it is advantageous to utilise short duration laser pulses in order to help reduce the energy levels required by the sensor systems. The use of such lasers helps achieve a greater range resolution.
Conventional sensor array detector elements used with active or pulsed light sources often have relatively long time constants which require shorter duration pulses to be integrated into longer pulses. This can lead to a reduction in the range resolution of the system as a whole. The use of active pulse light sources as a means of achieving greater range resolution has been made possible by the use of Q-switching, whereby laser sources can achieve nanosecond pulse durations. To help overcome the integration problems associated with conventional array detector elements, Avalanche Photo-Diodes (APD's) have been used. APDs can readily perform the required optical detection and processing of short duration pulses, but problems still exist in relation to the fabrication of APDs into arrays.
In a paper number #3065-04 presented at the SPIE AeroSense meeting (Apr. 20–25, 1997, Orlando, Fla.), a prototype active imaging laser-radar receiver was presented. The receiver incorporated an array of fibre coupled multi-channel receivers enables it to acquire images from a single laser pulse. Conventional scanned laser-radar imaging receivers require multiple pulses to assemble full images and suffer from jitter and image tearing caused by platform or target instability and other environmental effects. The paper proposed the use of a single pulse approach thereby eliminating distortions and providing high quality, high speed range based images.
The receiver as presented in the paper consisted of a focal plane array, formed of end polished multi-mode fibres. Each fibre acts as a light bucket, thereby capturing optical signals and relaying said signals to a series of detector elements. An array of APDs (APDs) was then utilised to detect and process light captured by the pixels formed by each end polished fibre-optic.
The configuration of the imaging laser-radar receiver as presented in the above referenced paper, requires that each pixel in the fibre-optic array has an associated APD detector. It therefore follows that, for example, a 24×24 array of pixel elements would require a total of 576 APD detector and processing elements. This makes any such a receiver comparatively large, and expensive in terms of the number of APDs and the associated electronics. In addition to the physical size and cost of developing such a system, the APD detection and processing electronics will remain largely dormant when a typical pulse repetition rate of 1 kHz is used. This follows because the detector is required to respond to pulses of a few nanoseconds duration, thereafter lying dormant for the remainder of the one millisecond pulse duration.
The invention provides for an imaging laser-radar receiver which requires substantially fewer detectors (and associated processing electronics) by utilising fibre-optic delay lines to supply time shifted pulses into each detector. The reduction in the number of detectors can provide for a physically smaller and more compact receiver system along with a corresponding reduction in the costs associated with the number of APDs required. Additionally, the invention provides flexibility in relation to the physical location of both detectors and associated electronics, thereby providing for further benefits in terms of packaging volume and the use of otherwise redundant space in host containers and vehicles.